Subject motion during a positron emission tomography (PET) study can seriously degrade image quality and affect the quantitative results. A number of methods are in use to restrain patients during brain PET scans; however, all but the most severe schemes are inadequate for completely eliminating head motion. For scanners with high spatial resolution, this motion can be a significant determinant of the achievable spatial resolution in the image. We have investigated the use of a real-time, ancillary measurement of patient head movement to correct for motion distortion in brain PET images. Past work included the development of a method to measure patient motion that can be operated while the patient is in the scanner with minimal patient disturbance, and testing in normal volunteers against a highly accurate spacetracking device to confirm its accuracy and reliability. The device is undergoing alignment measurements to the scanner in anticipation of characterization studies with phantoms and patients. The next stages of the project include (1) validation of the device in phantoms and patients undergoing scans on the Scanditronix PC2048-15B brain PET scanner with post-acquisition correction of the image data, and (2) modification of the applications software and testing of the device for use in the newly acquired GE Medical Systems' Advance whole-body PET scanner. The motion correction measured by the device will be compared with the results of an algorithm based on the PET data themselves.